This invention relates to medical syringes. It is concerned with those where a liquid part of a medical substance is initially kept in a syringe while another part, which may be in liquid or powder form, is in a separate capsule. A transfer device couples the syringe and capsule, and in a preliminary operation the liquid in the syringe is conveyed through its piston-seal to the capsule, where it mixes with the contents, and then back in ready-to-use mixed form to the syringe. The transfer device is decoupled, and a piston rod can then be fitted to the piston seal.
Various arrangements are described in WO 00/13723. However, there is a risk, at the end of the transfer process and when decoupling the transfer device from the syringe, of pulling out the piston seal, and it is the aim of this invention to reduce or possibly eliminate that risk.
According to the present invention there is provided a syringe assembly comprising a syringe with a first liquid medical substance and a transfer device connectable thereto for preparatory mixing of said first substance with a second medical substance in a capsule connectable elsewhere to the transfer device, the syringe having a piston-seal with a screw-threaded socket facing rearwardly to the open end of the syringe, and the transfer device having a needle projecting from its leading end which screws into said recess while the needle penetrates through the piston-seal into the first substance, wherein a cap is slidable axially up to a forward limit but is non-rotatable with respect to the transfer device which extends co-axially through it, and wherein the cap has screw-threaded engagement with the rear end of the syringe, the arrangement being such that, when the transfer device is offered up to the syringe and the cap at its forward limit is screwed thereto, the transfer device, rotating with the cap, screws into the piston-seal and completes its attachment thereto before the completion of the cap and syringe attachment, and such that the removal of the transfer device is by unscrewing the cap, which frees the transfer device from its engagement with the screw-threaded socket before the cap is freed from the syringe.
Generally, the syringe will have a capsule within a housing with which it is effectively integral, and it is the housing which will have the screw thread for engagement by the cap. In that case, the cap may penetrate into the rear end of the housing and, when screwed fully home, engage the rear end of the capsule to clamp that firmly within the housing.
The pitch of the screw threads may be such that both the cap and the transfer device move forward together in unison. But the axial length of the thread in the socket will then be shorter than the axial length of the cap thread. Alternatively, the socket may have a thread of relatively coarse pitch compared with that of the cap, so that the transfer device advances ahead of the cap until it is fully home in the piston-seal socket. The cap continues to be screwed on, and xe2x80x9ccatches upxe2x80x9d the transfer device.